Getting a free seat upgrade from Coach/Economy to Business class seams like a dream scenario unlikely to happen, but in reality it’s easy using one simple technique.
It takes a simple white lie. You should note that throughout this you need to maintain the most friendly, warm, understanding and empathetic body language and tone possible. It’s well known throughout the airline industry that losing “bad” or complaining customers isn’t a big deal - there is so much turnover that if a customer is complaining it’s better just to have them fly with someone else.
This works as long as your plane has screens on the seatbacks, which makes this ideal for long-haul flights. Simply ring for an attendant, and politely let them know that the headphones on your seat/screen don’t work.
They’ll bring you another set of headphones. “Try them out” and politely let them know this pair doesn’t work either. It helps to act a little and be understanding - “I’m really sorry for bugging you about this”, etc. It’s like waiters in a restaurant - they’re so used to assholes that when someone strangely polite comes along they’ll bend over backwards to help you.
At this point one of two things will happen. Most likely you’ll be taken up to an empty seat in business class, or if it’s not a full flight (unlikely for trans-oceanic) offered to relocate somewhere else in the economy cabin. If it’s the latter, tell them that you prefer the emergency exit row/window seat/aisle seat you currently have. At this point they’ll move you up to business, or feed you free drinks for the rest of the night. Not bad for 5 minutes of dialogue